Method for manufacturing of sound killed steel ingots

ABSTRACT

A high quality homogeneous sound killed steel ingot having a reduced quantity of negative segregation and of inverted V-shaped segregation zones is produced by adding a stirring agent to the molten steel during the solidification process and at a time period between the period during which the negative segregation section and the inverted V-shaped segregation zones are formed, and the period during which the V-shaped segregation zone is formed wherein the stirring agent is preferably an element, alloy, compound or mixture thereof of an element selected from the group of elements of Groups I-a or II-a of the Periodic Table or zinc, or the halides of an element of Groups I-b, III-b, IV-a, IV-b, VI-a, VII-a, or VIII.

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[451 Apr. 3, 1973 I METHOD FOR MANUFACTURING OF SOUND KILLED STEELINGOTS [75] Inventors; Kilehl Nat-its, ltarni; Tnkanlke Mori, Ashiya;Takamlehl Itoo, Kobe, all of Japan [73] Assignee: Kobe Steel, Ltd,Kobe-city, Japan [22] Filed: (May 8, 1970 211 Appl. No.: 35,842

[30] Foreign Application Priority Date May 8, 1969- .lspsn ..44l35364[52] US. Cl. "164/58 [511 Ill. Cl. ..B22d 27l20 [58] Hello! Searchl64/55-59 [56] Relerelees cm UNITED STATES PATENTS 2,837,800 6/1958lhchiya et al 164/56 3,208,117

9/1965 Goedeeke eta] ..l64/56 FOREIGN PATENTS OR APPLICATIONS PrimaryExaminer-Robert D. Baldwin Assistant Examiner-John E. RoethelAttorney-Oblon, Fisher & Spivak [57] ABSTRACT A high quality homogeneoussound killed steel ingot having a reduced quantity of negativesegregation and of inverted V-shaped segregation zones is produced byadding a stirring agent to the molten steel during thep 4 Claims, 6Drawing Figures 1/1964 France ..l64/56 6/l95l Belgium ..l64/56 pPATENTEDAPR 3 I375 FIG. 2b

INVENTORS KHCHI NARITA TAKASUKE MORI TAKAMICHI T00 04w, Au 4 spwwATTORNEYS METHOD FOR MANUFACTURING OF SOUND KILLED STEEL INGOTSBACKGROUND OF THE INVENTION 1. Field Of The Invention This inventionrelates to a method for manufacturing a sound killed steel ingot and toproducts produced therefrom.

2. Description Of Prior Art Killed steel excels rimmed steel orsemi-killed steel due to its greater degree of interior homogeneitywhich permits its use as a high grade structural steel or a forgingsteel which requires severe standards. However, although killed steelhas excellent homogeneity, except for the presence of solidificationshrinkage holes occurring at the upper portion of the ingot, segregationdefects, resulting from a solidification segregation phenomena are oftendetected in the ingot. For instance, negative segregation sections mayoccur at the center of the bottom side of the ingot. Inverted V- shapedsegregation zones may occur near the center portion of the upper halfofthe ingot and V-shaped segregation zones may occur at the center ofthe ingot.

' Oxide type inclusions of relatively large size are often found in thenegative segregation sections and it is these inclusions which are thefrequent cause of defective steel. Sulphide type inclusions ofrelatively large size are frequently found in the inverted V-shapedsegregation zones which has the effect of deteriorating the workabilityand toughness of the steel.

In recent years, the demand for large size high quality structuralsteels have increased considerably. This increased demand hassignificantly magnified the segregation problem, since large sizesnecessitate longer solidification periods and the longer the period ofsolidification, the greater is the degree of segregation and the largerthe defects.

Various attempts have been made by the prior art to minimize thesolidification segregation phenomena. For instance, it has beensuggested to accelerate solidification by increasing the pressure on themolten steel during solidification so that initial solidification willoccur at a higher temperature. Heat is removed rapidly so as tohomogenize the structure in the steel ingot and reduce segregation.Another technique suggested in the prior art is homogeneous casting (HOCMethod) or ultrasonic casting wherein the ingot is reduced and thestructure is homogenized. Still another prior art method is supersoniccasting, wherein the crystal grains within the ingot is pulverized, thehardness of the interior structure is increased and the cavities arereduced. Another method is the scraper method, wherein after casting themolten steel, it is solidified while stirring so that the boundarybetween the dendride section and the free crystal section can be definedwithout formation of V-shaped or inverted V- shaped regeneration. Bythis latter technique, a fine interior structure steel ingot can beobtained, the segregation of phosphorus, sulphur, oxygen, etc. isreduced and the mechanical properties of the central section of thesteel ingot is improved. Another technique is electromagnetic stirring,wherein the steel ingot is solidified in a rotating magnetic field andthe inverted V-shaped segregation is removed to the center by thebuoyance in the centrifugal field. The crystals are then pulverized.Another method is the rotary casting method, wherein the growth ofculumnar structure section is suppressed and the tesseral system zone isincreased to provide a homogeneous ingot. Still another method is vacuumcasting wherein casting is carried out under vacuum to reduce gasses inmolten steel and thereby minimize the extent of defects growing in thesteel ingot, etc.

All of these ingot-producing methods for producing killed steel ingotsof homogeneousquality are in principle simply variations of one or moreof the following techniques:

l. Controlling macroscopic mass transfer in molten steel duringsolidification so as to homogenize and compact the structure of theingot;

2. Quicken solidification so as to reduce solidification segregation andcompact the structure; and,

3. Reduce the absolute amount of H and O which often causesolidification defects when the degree of segregation is great. The mostindustrially successful technique, however, has been the vacuum castingmethod, which uses the principle (3) of reducing the quantity of H and0. All of the other methods cause difficulties for existing plantfacilities and accordingly are only used for specialty purposes.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide a high quality killed steel in large quantities.

It is another object of this invention to provide a high quality killedsteel which has a reduced quantity of segregation sections.

Another object of this invention is to reduce the quantity of negativesegregation sections in killed steel.

Still another object of this invention is to reduce the quantity ofinverted V-shaped segregation zones in killed steel ingots.

These and other objects have :now herein been attained by adding astirring agent to the molten steel during the solidification processbetween the time of the negative segregation section and invertedV-shaped segregation zones are formed, and the time that the V- shapedsegregation zone is formed. Suitable stirring agents include alloys,compounds or mixtures of ele' ments of Groups I-a, or II-a of thePeriodic Table or zinc or the halides of an element: of Groups I-b,III-b, IV-a, IV-b, VI-a, VII-a, or VIII.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In this invention, themolten steel stirring agent must have the following properties:

l. The boiling temperature must lie between 450" and 1,500 C. It theboiling temperature is lower than 450 C., the stirring action is toovigorous which is dangerous in practical application. If the boilingtempera- The alloy as the molten steel stirring agent comprises twogroups, one of which is alloys of the mutual alloys among the elementalsubstances mentioned above and the other is alloys containing otherelements such as Cu, Al, Si, Ti, Zr, Nb, Ta, Mn, Fe, and Ni. Forinstance, the alloy belonging to the first group may be any of thefollowing or mixture thereof:

Ca-Zn, Ca-Mg, Ca-Li, Ca-Na, Li-Mg,

K-Na, Li-Na, K-Mg, Ca-Mg-Zn K-Mg-Na and the second group are as follows:

Al-Ca, Ca-Cu, Ca-Si, Ca-S, Ca-Ni,

Al-Mg, Cu-Mg, Ni-Mg, C-Mg, Mg-Pb,

Mg-Mn, Mg-Ti, Mg-Si, Mg-S, Mg-Zr,

La-Mg, Ce-Mg, Cr-Mg, Al-Zn, Cu-Zn,

Ti-Zn, AlLi, Al-Na, Na-Si, Fe-K,

Cu-K, Cu-Na, Cr-Na, Al-K, Al-Cu-Mg,

ALMg-Si, K-Si-Mg, Al-Mg-Zn, Fe-Si-Mg, Mg-Si-Ca,

K-Si-Na, Mg-Si-Ni, Mg-Si-Cu, Al-Cu-Zn, Mg-Ni-Ca,

CuNi-Z-n, Mg-Si-Ca-Fe, Mg-Si-Ni-Fe. The compound as the molten steelstirring agent comprises inorganic halides, hydroxides, carbonates,bicarbonates and oxides of the La group and halides of the lb, lI-a,II-b, III-b, lV-a, IV-b, Vl-a, VII-a, and VIII Groups. Halides:

KCl, CsCl, NaCl, LiCl, Kl, Csl, Nal, CsBr, NaNr, LiBr, KF, CsF, CuCl,CuBr, CuBr BaCl Becl tends to form oxides to a high degree at this stagewhich cannot move as freely as carbon and sulphur. The oxides,therefore, accompany the settling ferrous crystals downward. At thelower half of the steel ingot, which corresponds to the negativesegregation section, the rate of solidification from the bottom isreduced and solidification is apparently stagnant. Ferrous crystals ofhigh purity and the oxides descend from the upper part and areaccumulated in this portion. The oxides grow by aggregation in thisarea, and part of the oxides will tend to float upward. At the upperhalf of the steel ingot, molten steel of a relatively low specificgravity, which is rich in the light elements, produced in thesolidification transition layer, move with the aggregation toward thezone of lower solidification phase rate, threading through thedendrites. Upon reaching the zone of sufficiently small solidificationphase rate, the

MgCl CaBr MgBr Cal Bel ZnCl,, lnF AlF LaCL- crcla, CfCl CIFz, PbClz,PbF2, Pblz,

M00 MoCl MoCl MnCl c001,, recs, NiCl,,

- close to equilibrium so that molten elements are briskly separated inmolten steel. Accordingly, at this period in the solidification process,relatively high purity ferrous crystals are produced in a solidificationtransistion layer. Carbon, phosphorous, sulphur and like substances areseparated in the remaining molten steel causing a local increase inconcentration of these like elements. Ferrous crystals of relativelyhigh purity have a large specific gravity which will settle downward,while molten steel rich in carbon, phosphorous, sulphur, etc. aggregateand will tend to float upward due to the difference in specific gravity.in the middle of solidification, the sedimentation phenomena of ferrouscrystals and floatation phenomena of molten steel rich in impuritiesoccur. Oxygen within the molten steel low specific gravity molten steelascends substantially along the solidification line, partially beingarrested by the crystals, forming the inverted V-shaped segregationline. As solidification advances further, the molten steel temperatureis lowered still further, the viscosity increases, and the percentage ofthe solid phase is increased. During this period, macroscopic-masstransfer in the remaining molten steel almost disappears. During thisperiod of advanced solidification, the V-shaped segregation line whichcorresponds substantially to the shape of the solidification line isformed.

In order to reduce the segregation sections such as the negativesegregation section and the inverted V- shaped segregation zone, it hasbeen found that homogenization can be attained by stirring the remainingmolten steel, after the middle stage of solidification, when thesegregation sections are formed.

In the present invention, negative segregation and inverted Vsegregation occurs, then the stirring agent is inserted so that thesesegregation zones are reduced and eliminated by homogenization.

This can be accomplished by using the fairly high vapor pressure in thesolidification temperature range of the stirring agent. In practice, asuitable stirring agent in the form of an element, alloy, compoundormixture thereof, is added to the molten steel so that the vaporizationof the stirring agent will stir and homogenize the molten steel.

Although the stirring agent may be any substance containing theabove-mentioned elements, for practical application, a Mg alloycontaning less than 50% Mg, Ca and the alloy thereof are desirable. Thestirring agent can be easily and effectively added to molten steel byattaching a suitable capsule containing the stirring agent to an ironrod, which is then inserted to the predetermined depth in the mold. Theamount to be added depends on the size of the steel ingot, but the useof amounts in excess of about 1% is apt to dangerously scatter moltensteel. An amount of less than about 0.000196 will not exhibit asufficient effect. The

- preferred amount is such that when the stirring agent is added, thesurface of molten steel at the feeder head section will shake'slightly.Excellent resultsare obtainable within the range of about 0.0005 to 0.02percent.

The metal element inserted into the molten steel will quickly evaporateand the vapors produced will stir and homogenize the molten steel,reduce the segregation and compact the cast structure. It will also havethe effect of accelerating solidification by removing evaporation heatfrom molten steel when the metal is vaporized. It will have the furthereffect of scattering the fine deoxidated products within the steel ingotthrough the evaporation of the metal producing numerous solidificationscores to produce a finer cast sturcture. As can be readily appreciated,therefore, the process of the present invention provides very desirableadvantages.

For best results, the molten steel stirring agent should be addedbetween the time slightly before the complete formation of the columnarstructure of the steel ingot and the time of formation of V-shapedsegregation zone is completed.

If the stirring agent is added too soon after casting, the fluidity ofmolten steel will be too high and the segregation will be rather small,so that the desired effects will not be obtained. Moreover, if added toosoon, the rather thin initial shell on the surface of the steel ingotcan easily be broken, allowing the molten steel to exude through to forma double layer. On the other hand, if the stirring agent is added closeto the complete solidification point, it will have very undesirableeffects. In fact, even greater solidification shrinkage holes may beformed and segregation and rolling up of scums may actually increase.

The amount of the molten steel stirring agent and the number of dosesdepend on the kind of stirring agent used, its composition, the chemicalcomposition of steel, the size of steel ingot, solidification time,etc., but in the ordinary ingot-making, one to five doses is sufficientfor this purpose.

FIGS. 1, 2, and 3 are macrographs for comparison of corroded structuresof steel ingots. In each FIG., a shows a comparative example and b showsthe example of the invention.

Having generally described the invention, a further understanding can beobtained by reference to certain specific examples which are providedherein for purposes of illustration only, and are not intended to belimiting in any manner.

EXAMPLE 1 835C steel (C; 0.37 percent, Si:'0.32 percent, Mn: 0.65percent, S: 0.018 percent, P: 0.013 percent, 0.0071 percent) which ismelted in a basic highfrequency furnace was top-poured into 100 kg sandmolds which were lined with an exothermic heat conservative. The castingtemperature was l,569 C. and the pour rate wasl ton/3.0 min. Uponcompletion of casting, the exothermic heat conservative was added to thefeeder head at the rate of Skg/t. One of the ingots cast as above wasallowed to solidify as it was for comparison purposes while to the otheringot was added Fe- Si-Mg alloy containing 20% Mg, packed in-a capsule.The ingot was treated three times, 9 min., 14 min., and '19 min. aftercasting at the rate of 200 g/t each time, by inserting the capsulethrough a guide into the center of the mold. Where sand molds are usedfor casting, the negative segregation section and the inverted V-shapedsegregation zone were formed inthe steel ingot within from about min. toabout 23 min. after casting. The

time to add the alloy was therefore predetermined to match the time atwhich these segregation zones were formed. V

The macroscopic corroded structures in the longitudinal section of thesesteel ingots are shown in FIG. 1, wherein a shows a macrograph of thecorroded structure of the steel ingot solidified without any addition.The branched dendrite zone is observed around the periphery of the steelingot, and several stripes of the inverted V-shaped segregation linesare recognized in this zone. On the other hand, b shows macroscopiccorroded structure of the steel ingot to which was added the Fe-Si-Mgalloy during the solidification process thereof, and substantially thewhole surface, except the surface layer section consists of finetesseral system. The inverted V-shaped segregation line is not recognized. The cast structure is highly dense and homogeneous.

The segregation of various elements in said two steel ingots are shownin comparison in Table 1 below:

TABLE 1 Comparison of Segregation Percenta e In 100 kg; Steel 8 C S P I0 Control Steel Ingot 12.5 15.2 13.9 20.2 Steel Ingot of Invention 5.55.0 4.3 9.1 I

Segregation Percentage {Maximum analytic value (average) in steelingotl/Average value As evident from Table 1 above, by application ofthe method of the'invention, steel ingots having a lesser degree ofsegregation of carbon, sulphur, phosphorus, oxygen, etc. can beobtained.

EXAMPLE 2 i S40C steel (C: 0.39 percent, Si: 0.27 percent, Mn:

0.71 percent, S: 0.023 percent, P: 0.019 percent, 0: 0.00065 percent)which had been melted ina basic high-frequency furnace was top-pouredinto the kg sand molds. The pouring temperature was l, 555 C. and thepour rate was 1. ton /3 min. 20 sec. Upon completion of pouring theexothermic heat conservative was added to the feeder head at the rate of5 kg/t. One of the steel ingots cast as above was left to solidify as itwas, as a control while an Al-Zn alloy containing 20% Zn, packed in acapsule, was added to the other ingot in two doses at 5 min. and 10min.after casting at the rates of 200 g/t and g/t, respectively. The alloywas inserted in the form of a capsule which was atare shown in FIG. 2,wherein a shows the steel ingot which was solidified without addition ofthe alloy, in which the inverted V-shaped segregation lines thoughslight are observed in the branched dendride section, and theV-shapedsegregation lines are observed at the central section. At thecenter of the lower half section of the steel ingot there exists atesseral system showing the negative segregation of carbon, sulphur,phosphoorus, etc. and the positive segregation of oxygen. b shows themacroscopic corroded structure of the steel ingot to which was added theAl-Zn alloy during the solidification process thereof, whereinsubstantially the whole surface of the steel ingot except the peripheralsection is observed to consist of fine tesseral system. Neither theinverted V-shaped segregation lines nor V-shaped segregation lines areobserved. The cast structure is notably dense and homogeneous. Thesegregation percentages of various elements in the two steel ingots arecompared in Table 2 below:

TABLE 2 Comparison of Segregation Percenta es in 100 kg Steel Ingots S PControl Steel Ingot 11.1 20.8 14.3 17.5 Steel Ingot of Invention 2.5 7.510.6 8.2

Segregation Percentage [Maximum analytic value (average) in steelingot]/Average value The steel ingot of the invention above had asmaller S30C steel (C: 0.29 percent, Si: 0.27 percent, Mn: 0.68 percent,S: 0.016 percent, P: 0.012 percent, 0: 0.0057 percent) which had beenmelted in a basic highfrequency furnace was top-poured into 1 ton sandmolds. The pouring temperature was 1,510 C. and the pouring rate was 1ton/1 min. 50 sec. After casting 5 kg of the exothermic heatconservative was added to the feeder head. The cast steel was in a roundshape, 45 cm average diameter and 95 cm long. The feeder head weighedaBout 1.2 tons. Two steel ingots were cast under the same conditions.One of the ingots was left to solidify per se for preparing a controlsteel ingot. Metallic Ca was inserted into the other ingot in the formof a capsule attached to the end ofan iron rod to the depth of cm abovethe upper solidifying surface in the mold. The ingot was treated threetimes during the solidivication process, in doses of 200 g, 150 g and100 g, min., 26 min., and 38 min. after casting, respectively. Thenegative segregation section and inverted V-shaped segregation zone wereformed in the steel ingot between from about 10 min. to about 90 min.after pouring. Accordingly, the Ca was added during the first half ofthe period in which these segregation zones were formed.

In the mascroscopic corroded structure of the steel ingot which wassolidified without any metal addition was recognized the invertedV-shaped segregation lines, V-shaped segregation lines and negativesegregation section. In the negative segregation section, a max- I imumsize of silicate of about 350 p. was observed to exist. On the otherhand, in the steel ingot to which Ca was added, the inverted V-shapedsegregation lines were reduced to a minimum, the V-shaped segregationlines disappeared and the oxides present in the position correspondingto the negative segregation section were dispersed in a minute form. Themaximum size thereof being about 50 u.

As shown in Table 3, it is evident that the steel ingot to which themethod of the invention was applied shows a lesser degree of segregationof various elements than'the ordinary steel ingots.

TABLE 3 Comparison of Segregation Percenta es o 1 on Steel Ing0ts( C S P0 Control Steel Ingot 76.2 93.8 81.0 69.2 Steel Ingot Of Invention 29.113.5 26.3 27.1

Segregation Percentage [Maximum anal tic value (average) in steelingotl/Average alue EXAMPLE 4 S40C steel (C: 0.44 percent, Si: 0.26percent, Mn: 0.67 percent, P: 0.009 percent, S: 0.019 percent, 0: 0.0028percent) had been melted in a basic open hearth was bottom-poured into4-ton metal molds. One of the steel ingots of the same surface plate wassolidified per se, as a control while to the other steel ingot wastreated according to the methods of this invention. Fe-Si-Mg alloycontaining 30% Mg was packed in a capsule which was attached to an ironrod and was inserted through a guide to the center of the mold in dosesof 250g, 200g, and 150g, 15 min., 25 min., and 35 min. after casting,respectively. The alloy was inserted to 15 cm above the solidifyingsurface at the time of the addition.

The macroscopic corroded structues of the two steel ingots are comparedin FIG. 3, wherein'a shows the control steel ingot and b shows the steelingot to which the methods of this invention were applied. In a, theinverted V-shaped segregation lines and V-shaped segregation lines aredistinctly observed, whereas in the steelingot b of the invention, theinverted V-shaped segregation lines are reduced to minimum, and the V-shaped segregation lines have completely disappeared. The density ofthecast structure of the steel ingot is increased throughout the ingot.

EXAMPLE 5 S40C steel (C: 0.38 percent, Si: 0.32 percent, Mn: 0.65percent, S: 0.014 percent, P: 0.011 percent, 0: 0.0042 percent) had beenmelted in a basic electric furnace was bottom-poured into 3 ton molds.The pouring temperature was l,550 C. and the pouring time was 4 min. 30sec., four ingots were prepared per each surface plate. One of the steelingots was solidified without any addition as a control. To seven othersteel ingots the method of the invention was applied, and uponcompletion of pouring, NaCl, ZnCl,, MgCl,, KF, Al-Ca alloy of 50% Ca,Fe-Si-Mg alloy of 3.0% Mg and a mixture of Cu-Zn alloy of Zn and NaCl,respectively, packed in capsules were inserted into the molds in threedoses at 15 min., 25 min., and 36 min. after pouring. These steel ingotswere rolled into blooms, 200 mm. dia., and sectional samples were takenfrom positions corresponding to the head, center and bottom of the steelingot, .and the macroscopic corroded structures of the respectivesections and the segregation of the components thereof were examined.

As is evident from the examples, when using the methods of thisinvention, the inverted V-shaped segregation zone and V-shapedsegregation zoneare reduced or eliminated, and large oxide typeinclusions occurring in the negative segregations section are turnedinto finer form. A markedly dense and more homogeneous cast structurecan be obtained; also, the segregation of the component elements isreduced and the segregation condition of oxygen is notably improved.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit and the scope of theinvention. Accordingly,

What is claimed and intended to be covered by letters patent is:

1. In a method for reducing a sound killed steel ingot,

the improvement comprising adding a stirring agent to the molten steelduring the solidification process of the killed steel ingot at a timeperiod between the period before the V-shaped segregation zone is formedand a after the negative segregation section and the inverted V-shapedsegregation zones are formed, wherein said stirring agentischaracterized by a boiling temperature of between 450,and 1,500 C., suchthat when introduced into the'molten steel, it will vaporize under,

the process conditions whereby the vaporsproduced willstir the moltensteel and thereby reduce the extent of negative and inverted V-shapedsegregation zones in said steel ingot.

2. The method of claim 1, wherein said stirring agent is selected fromthe Group consisting of:

1. Elements, alloys, compounds or mixtures of at least one elementselected from the Group consisting of elements of Groups I-a, ll-a, orzinc,

2 Halide compounds of elements selected from the Group consisting ofthose elements in Groups I-b,

III-b, IV-a, IV-b, Vl-a, VII-a and VIII of the

2. The method of claim 1, wherein said stirring agent is selected fromthe Group consisting of:
 3. The method of claim 2, wherein the stirringagent is a magnesium alloy containing less than 50 percent magnesium. 4.The method of claim 1, wherein the stirring agent is inserted directlyinto the ingot in capsule form in an amount of between 1 and 0.01percent.